Glad you brought this up since it provides a platform to possibly clarify what is considered soap-boxing by the site?
The first mod post you provide is a mod note where the mod states......."If you choose to make virtually every topic you post related to a particular subject, that is by definition, 'soap-boxing' ".
So yes, we see from this that soap-boxing for all intent and purposes on the site is where virtually every topic posted is related to a particular subject.
This doesn't state someone can't post on a particular subject. They just can't make most ( virtually ) of their post about a particular subject.
There is a big difference between some and most.
You, and others of a particular group, attention is focused on ANY post by Archaic on a particular subject. You and this group are misusing the term.